Automatic clutch systems with wet or dry clutches which, to transfer torque, are held by spring force in the engaged or closed condition and are changed by way of a pneumatic, hydraulic or electric actuator to a disengaged or open condition, have been known for a long time. Owing to the comparatively high component complexity and to the path control necessitated for the then usual spring characteristics, clutch systems so formed are relatively expensive. However, when used in vehicles they have the great advantage that in the event of a failure in the electric, pneumatic or hydraulic system for controlling and actuating the clutch during driving, an engaged drive train is maintained which, in such a case, is the safe and desired operating condition to be aspired to.
On the other hand, automatic clutch systems are also known in which, when not actuated, the clutch is kept in a disengaged condition and can be changed to the engaged or closed condition by the application of pneumatic, hydraulic or electric forces by way of at least one actuator. The disengaged condition can be supported by a spring force.
Actuation of such a clutch can be controlled comparatively easily and inexpensively with an actuator in the form of a piston-cylinder arrangement. Here, the clutch is disengaged by venting or emptying the clutch actuator, whereas to engage the clutch, the clutch actuator is filled with a pressure medium. During this, the pressure is usually controlled by an electromagnetic control and regulation unit. The pressure control during clutch actuation also makes it possible, with the help of the clutch, to transmit various torques in slipping operation. Such clutch systems or clutches, with pressure control by way of known switching values, are known from DE 22 22 184 A1, DE 100 18 677 A1 and WO 2004/053349 A1.
As already mentioned, the switching valves for clutch actuation are electrically controlled by an electronic control and regulation unit. However, if the motor vehicle's voltage supply should fail, proper control of those valves can no longer be guaranteed and with conventional clutch systems, the operating behavior of the motor vehicle can become unforeseeable for the driver.
Thus, for the operation of automatic clutch systems, the desired operating behavior is such that, when the motor vehicle is driving, the shifting system is inactive (a gear is engaged) and the clutch control system fails, the drive train remains in an engaged condition and the clutch in an engaged condition. Moreover, the operating behavior of the automatic clutch system, when the motor vehicle is stationary, the engine is running and a starting gear is engaged, should be such that in the event of clutch control failure, with the help of the clutch the drive train is disengaged and the clutch brought to or kept in a disengaged condition.
Against this background, the purpose of the present invention is to provide a comparatively simple and inexpensive automatic clutch system with a “pressure-engaged” clutch which, in the event that the voltage supply for the clutch control system should fail, will ensure that during driving and when shifting, is inactive the full clutch torque or at least a defined part of it is transferred, and which, when the motor vehicle is stationary, the engine is running and a transmission gear is engaged, will prevent the spontaneous engagement or engaging of the clutch.
In this context a “pressure-engaging” clutch is understood to be a clutch of the type described above, such that the clutch is engaged and torque-transferring when an actuating force is applied to it via at least one pressure-operated actuator.